1. Field of the Invention
The present invention relates to color correction matrix determining method and apparatus and image photographing method and apparatus using the color correction matrix determining method and apparatus. In particular, the present invention relates to a correction matrix determining method and apparatus that determine a color correction matrix used to perform image conversion in a color image photographing system such as a digital camera. The present invention also relates to image photographing method and apparatus that improve light source dependency in a color processing system of the image photographing apparatus that converts input image data, such as image data obtained through photographing with a CCD, into output image data such as image data for display on a monitor.
2. Description of the Related Art
In usual cases, in a color image photographing system such as a digital camera, a scene to be photographed is photoelectrically read by an image sensor such as a CCD sensor, and a photographed color image is displayed on a monitor such as a liquid crystal display.
In this manner, the digital camera ultimately outputs an output image as an RGB signal for monitor output or as a luminance color difference signal (YCC) by performing JPEG compression. In the case where the output image is outputted as an RGB signal for monitor output, the RGB signal that should be outputted is determined by specifications. However, there are various kinds of CCDs that are usable in a digital camera, so that it is required to perform image conversion from an RGB signal of the CCD into an RGC signal for a monitor
An example construction of a color processing system of a conventional digital camera is shown in FIG. 9.
In the color processing system of the digital camera shown in FIG. 9, two color correction matrixes that are a first matrix (linear matrix L-MTX) 92 and a second matrix (chroma matrix C-MTX) 98 are arranged so that a white balance circuit 94 and a γ circuit 96 that respectively perform white balance correction and γ correction on image data read from a CCD 90 are sandwiched therebetween.
As described above, in the color processing system of the conventional digital camera shown in FIG. 9, there are provided the two color correction matrixes. The first matrix (L-MTX) 92 is applied to image data (RGB signal) from the CCD 90 in order to perform the white balance correction and the γ correction. Following this, the image data is processed again using the second matrix (C-MTX) 98, thereby converting the RGB signal of the CCD 90 into an RGB signal under the BT709 that is a recommended standard for HDTVs and has been stipulated by ITU-R (International Telecommunication Union-Radio Communication Sector). Here, the white balance correction for removing a color tint of a photographing light source is generally realized by gain adjustment on a specific color space.
The white balance correction through the gain adjustment gives an appropriate result in many cases, although it is impossible to completely remove the color tint of the light source depending on the kind of the light source. This results in various problems. For instance, in the case of a skin tone photographed under a fluorescent lamp, there remains a cyan-green tint even after the white balance correction is performed, so that it is impossible to reproduce a color like the skin color observed under sunshine. This phenomenon is called the “light source dependency”.
In the color processing system of the conventional digital camera shown in FIG. 9 that uses the two color correction matrixes that are the L-MTX 92 and the C-MTX 98, these color correction matrixes are not produced by sufficiently giving attention to such light source dependency, so that there remains a problem in that the light source dependency of the conventional digital camera is not sufficiently improved.
It should be noted here that there is a conventional digital camera that uses only one color correction matrix that is, for instance, the second matrix (C-MTX) 98. In a color processing system of such a digital camera, there is also a problem in that its light source dependency is not sufficiently improved.
That is, there is a problem in that there has conventionally been known no special technique of improving the light source dependency described above.
Also, in the color processing system of the digital camera shown in FIG. 9, there are provided the two color correction matrixes (the first matrix (L-MTX 92) and the second matrix (C-MTX 98)). The characteristic is uniquely determined in the case where these two matrixes are both applied, although it is possible to arbitrarily determine each matrix.
However, it has conventionally been uncertain which role is played by each of the matrixes L-MTX 92 and C-MTX 98. Consequently, there is a problem in that there is not fully utilized a characteristic of the construction where two color correction matrixes are provided in the manner described above.